1. Field of the Invention
The present invention relates to the preparation of glycidyl polyethers by reaction of epoxy compounds, notably the 1-halo-2,3-epoxyalkanes, with the alkali metal salts of polyhydric phenols.
2. Description of the Prior Art
The glycidyl polyethers are commonly denominated "epoxy resins". Such resins constitute a class of thermosetting polymers characterized by the presence of recurring oxirane, i.e., ##STR1## and/or pendant hydroxyl linkages which, after cross-linking, provide systems having a wide variety of different useful properties, useful in a wide variety of different fields, i.e., as coatings, finishes, adhesives, impregnants for electrical components, etc. The immediately aforesaid, moreover, has favored development of such resins along widely diverse lines of application.
Most prominent among the "epoxy" resins is the condensation product of bisphenol A [2,2-bis(4-hydroxyphenol) propane] with epichlorohydrin, a polymer very well known to the art and especially adapted for the coating and electrical fields.
The epoxy resins of bisphenol A/epichlorohydrin type are conveniently and typically prepared by condensing bisphenol A with epichlorohydrin in the presence of water and alkali. Indeed, the prior art is replete with such processes.
For example, U.S. Pat. No. 2,801,227 features a process for the production of glycidyl ether of a polyhydric phenol. This process entails the addition of an aqueous solution of alkali metal hydroxide to an agitated solution of polyhydric phenol in epichlorohydrin, in a proportion of at least 3 moles of epichlorohydrin per phenolic hydroxyl equivalent of the phenol. The water and a portion of the epichlorohydrin are simultaneously distilled from the reaction mixture.
In the process of the aforesaid '227 patent it is essential to control the rate of introduction of the alkali metal hydroxide solution and the rate of distillation, so that the concentration of water in the reaction mixture will be maintained at about 0.3 to 2% by weight of water. The reaction between the epichlorohydrin and the polyhydric phenol takes place at a temperature of the order of 120.degree. C. In practice, the total amount of the alkali metal hydroxide (sodium hydroxide) is added over the course of from 3 to 5 hours. The excess epichlorohydrin is then eliminated by distillation and a solvent is introduced into the reaction mixture so as to separate the resin from the salt (sodium chloride) formed.
Another type of process is based on the principle according to which the alkali plays a dual role in the synthetic scheme: a catalytic role to favor the condensation of the polyhydric phenol with the epichlorohydrin, but also the role of dehydrohalogenation agent, in order to transform the "chlorohydrin" groups into oxirane moieties. Hence, such processes are of two-stage type.
Thus, French Pat. No. 1,336,444 describes such a two-stage process, wherein one mole of bisphenol A and at least 10 moles of epichlorohydrin are contacted at a temperature on the order of 65.degree. C. in a first stage, by progressively or incrementally adding thereto a 50% aqueous solution of sodium hydroxide, over a period of from 2 to 4 hours, with the amount of sodium hydroxide introduced during this period being less than 16% of the total amount required, which amounts to 2 moles of the hydroxide per mole of bisphenol A employed. Subsequently, the water is eliminated by distillation in the form of its azeotrope with epichlorohydrin, and, therefore, only a portion of the excess epichlorohydrin is eliminated from the reaction mixture.
In a second stage, the dehydrochlorination is effected at about 99.degree. C., in the residual epichlorohydrin, by adding the remainder of the sodium hydroxide as the dehydrochlorination agent, in the form of platelets. After about one hour, the excess epichlorohydrin is distilled off.
The resin obtained is subsequently dissolved in a solvent, washed with water and two phases are obtained, an aqueous phase containing the sodium chloride formed and an organic phase containing the resin. The organic phase is separated and is again treated, for about one hour, at approximately 90.degree. C. with particulate sodium hydroxide. The resin is then recovered by means of such various operations as washing, neutralization and drying.
Nonetheless, the aforenoted known processes are extremely difficult to perform: same require relatively long reaction periods, exactingly strict controls over different reaction conditions and numerous stages for the purification and/or recovery of the resin desired. Also, the loss in epichlorohydrin which is necessarily used in excess, is far from negligible, even if the process be carried out with the greatest of care.
Recently, per U.S. Pat. No. 3,519,653, it has been shown that it is possible to prepare intermediates useful in the manufacture of complex epoxy resins, by reacting a suspension of the alkali metal salts of anhydrous dihydroxy phenols, with a large amount of epichlorohydrin. The epichlorohydrin again plays a dual role: it is, on the one hand, a reagent and, on the other, a solvent for the resin formed, thus rendering the reaction mixture more manipulable.
However, this technique, which is undoubtedly of fundamental interest, leads to liquid resins of good quality only with great difficulty. Because the alkali metal salts of dihydroxy phenols are insoluble in epichlorohydrin, the contacting of the two reagents is difficult; the reaction, thus, is relatively slow. The scale-up of such as process to the industrial scale is exceedingly problematical by reason of the large quantities of epichlorohydrin that are necessarily employed and which must be recycled in order not to sacrifice the economy of the process.
In view of the foregoing problems, certain authors, at Macromol. Chem., 179, 7, 1661-1671 (1978), have proposed to effect the synthesis of the resins in question from the alkali metal salts of dihydroxyl phenols and from 1-halo-2,3-epoxyalkanes in an anhydrous, aprotic and homogeneous medium.
These authors have determined that the decisive parameter in said synthesis is the total solubility of the alkali metal salt of the polyphenol in the reaction medium. Because these salts are insoluble in most organic compounds and, in particular, in epichlorohydrin, there was employed, in order to enable working in a homogeneous medium, large amounts of dimethylsulfoxide as the solvent. It was also found that only this compound was suitable for use in the synthesis in question, with the provision that the concentration of the alkali metal salt of the polyhydric phenol did not exceed a limiting value, which was 0.7 mole per dm.sup.3 in the case of the disodium salt of bisphenol A, at 80.degree. C. Upon completion of the reaction, the solvent must be eliminated in order to recover the resin, which appears to be difficult from the outset because of the high boiling point of dimethylsulfoxide; furthermore, the risk of degradation of the resin is not negligible. And it too has now been found that the resins produced in this manner have an unacceptable coloration in vis-a-vis their intended use and that their viscosity is generally high. It was also determined that under certain conditions a gel appears. The development of such a process, thus, involves exacting control over the reaction conditions and, in any event, the product resins can be said to be of inadequate quality.
Accordingly, there still exists a great need in this art for an effective, yet facile process for the preparation of glycidyl, phenolic polyethers. Such need is especially felt in the field of the liquid epoxy resins.